16 ON A NEW SPECIES OF ENTEROPNEUSTA, 



which have become enclosed during the formation of the skeleton, 

 and not cells which have immigrated later. 



" Chondroid tissue^'': As in the genus Ptychodera generally the 

 " chondroid tissue " of the proboscis neck is not greatly developed, 

 and the cell strands appearing generally in transverse sections as 

 small isolated masses are derived as Spengel has shown mainly 

 from the epithelium of the proboscis pockets. As in Pt. clavigera 

 a band of " chondroid tissue " continuous with the lateral tissue 

 is present between the "end plate" and "keel" of the proboscis 

 skeleton. The cell strands of this ventral portion are very richly 

 developed, and are derived from the epithelial cells lining the ventral 

 proboscis pockets and behind the posterior edge of the proboscis 

 septum from the continuation of the same epithelial cells lining 

 the ventral unpaired portion of the proboscis coelom. 



Heart-bladder : The heart-bladder is esentially similar in its 

 general relations to that of Pt. minitta. It is a completely closed 

 sac, having no connection either with the vascular system or with 

 the proboscis coelom. On its lateral walls the muscle fibres 

 belonging to the dorso-ventral muscle plate are very well marked 

 (fig. 5, dsc), but as in the other species of the genus they do not 

 possess a musculature of their own. On the ventral wall there is 

 present as in the described species a very distinct single layer of 

 transverse muscle fibres which, so far as I have observed, are 

 entirely confined to this wall. In this species the ventral wall of 

 the heart-bladder is infolded into the cavity of the bladder in a 

 very characteristic manner. In its posterior part the central 

 blood space of the proboscis is a transverse cleft between the 

 ventral wall of the heart-bladder and the proboscis gut just as in 

 the species previously described, and varies in size according as it 

 is filled with blood or empty. In its anterior region, however, 

 the ventral wall of the heart-bladder is infolded into the cavity 

 of the bladder along the median line so as to give rise to a tubular 

 cavity which communicates with the central blood space by a 

 narrow longitudinal slit (fig. 4, ivic). Then posteriorly to the 

 infolding by the gradual receding of the two edges of the slit, the 



